The present invention relates to a bit rate control method for a moving picture coding apparatus to produce the moving picture data in which the bit rate is different from the compression coded moving picture data.
Recently, according to the development of the digital signal processing engineering, moving picture data can be compressed and coded, and the compression and coded moving picture data is easy to handle. Further, with the development of a computer network, the compressed and coded moving picture data are increasingly transmitted and received through various transmission paths. For example, as a TV broadcasting station, a large amount of the moving picture data previously accumulated, is compression coded and saved, and as a VOD (Video On Demand), at the need of the user, the compressed and coded moving picture data can be transmitted onto the transmission path.
However, where the compressed and coded moving picture data is transmitted to the transmission path, it is possible that the bit rate which can be transmitted on the transmission path differs from the bit rate of the moving picture. If the bit rate of the moving picture data is high, when the moving picture is transmitted at that rate, there is a problem in that delay is generated in the reception data, and the moving picture data cannot be reproduced in real time. Accordingly, in order to reproduce a moving picture in real time, it is necessary that the bit rate of the moving picture be reduced. Further, it is necessary that the bit rate of the moving picture data also be adjusted to the receiving bit rate of the terminal equipment receiving the moving picture data. Further, even in the case where not the whole frame in the compressed and coded moving picture data, but only a portion of the frame is selected and removed and transmitted, when the bit rate of the selected and removed moving picture data exceeds the bit rate of the transmission path, processing must be conducted to the reduced bit rate. As described above, when the moving picture data is transmitted through various reception terminal equipment and transmission paths, it is necessary that the bit rate of the moving picture data be changed by adjusting to various bit rates, and a delay is generated in the data transmission by a time period for the processing of the rate control.
Further, when the VOD server sends out a plurality of moving picture data to a plurality of terminal equipment, as in the case of the VOD, the quantity of simultaneous connected terminal equipment is limited. This is because excessive rate control processing is required, burdening the VOD server. That is, the bit rate control method that quickly changes the bit rate of the moving picture data is absolutely necessary.
The conventional and well known technique of controlling the bit rate of compressed and coded moving picture data is to initially decode the data into non-compressed moving picture data and then to code it again, thereby changing the bit rate. However, in this method, excessive processing is required because the moving picture data is decoded and coded again. In addition, it is difficult to quickly change the bit rate and produce the moving picture data.
Further, as the conventional technique to lighten the re-coding processing and to increase the processing speed, a technique disclosed in JP-A-8-23539 is well known. In FIG. 38, the structure of the conventional moving picture coding apparatus is shown. In FIG. 38, a moving picture coding apparatus 5001 is structured by a variable length decoding means 5002 connected to an input means 5006, a re-quantizing means 5003, a variable coding means 5004, a buffer memory means 5005, and a buffer occupation amount detecting means 5006, and is connected to an output means 5008.
Next, an operation of the moving picture coding apparatus will be described. In FIG. 38, the input means 5007 inputs the coded moving picture data into the variable length decoding means 5002 for each one frame, and inputs the desired bit rate into the re-quantization means 5003. Then, the variable length decoding means 5002 conducts variable length decoding on the input data, and the quantized DCT (Discrete Cosine Transform) coefficient is found, and outputted to the re-quantization means 5003. Then, the re-quantization means 5003 re-quantizes the quantized DCT coefficient, and outputs it to the variable length coding means 5004. In this connection, the re-quantization means 5003 compares the bit rate inputted from the input means 5007 to the buffer fullness amount inputted from the buffer occupation amount detecting means 5006, and the quantization value is set so as to satisfy a predetermined bit rate, and the re-quantization is conducted. Herein, the quantization value means a value to divide the DCT coefficient is divided in the quantization. Further, the variable length coding means 5004 conducts variable length coding on the re-quantized DCT coefficient, and supplies the moving picture data which is variable length coded, to the buffer memory means 5005. The buffer memory means 5005 outputs the inputted moving picture data from the variable length coding means 5004 to the output means 5008, and outputs the data amount of the moving picture data to the buffer occupation amount detecting means 5006. After the buffer occupation amount detecting means 5006 adds the data amount and detects the buffer occupation amount, the total amount of the data is outputted to the re-quantization means 5003.
As described above, when, by using the moving picture coding apparatus 5001, the bit rate is controlled from the compressed and coded moving picture data and the new moving picture data is produced, the moving picture data is produced through the process in which the input moving picture data is once variable length decoded, re-quantized, and variable length coded. That is, because the moving picture data is decoded and coded again until the inverse quantization processing, the calculation load is large and it is difficult to quickly conduct the rate control. Further, the moving picture data is structured by a plurality of frames, and because, when the moving picture data is compressed and coded, in order to increase the efficiency, interframe predictive coding, which is the process of using the correlation between the given frame and the immediately preceding frame, is generally used. Then, when the moving picture data includes the frame on which interframe predictive coding is conducted, a problem exists when the re-quantization is conducted by using the moving picture coding apparatus.
In the interframe predictive coding, the frame (Pi) on which re-quantization in the moving picture coding apparatus 5001 is conducted, is used when it is timewise at (Pi+1), which is the frame immediately following Pi. The frame Pi is necessary for decoding the (Pi+1) frame.
Then, when the re-quantization is conducted in the moving picture coding apparatus 5001, because the re-quantization means changes the quantization value of the input data, the frame (Pi) prior to re-quatization differs from the frame (Pi′) after re-quantization. Accordingly, when Pi′ added to (Pi+1) and the (Pi+1) is decoded, the decoded image is deteriorated because the difference exists between the Pi to be originally added and Pi′. Hereinafter, the difference between the Pi and Pi′ is called as the motion compensation error. That is, for moving picture data on which inter-frame predictive coding has been conducted, when the moving picture data is produced by using the moving picture coding apparatus 5001, image quality deterioration is caused due to the motion compensation error. Further, in order to prevent the image quality deterioration, it is necessary that the frame next to the frame on which the re-quantization is conducted, is re-corded including the motion compensation, and there is a problem that the processing time is further increased.
In the conventional moving picture coding apparatus, when the bit rate of the moving picture data which is previously coded, is changed, and the new moving picture data is newly produced, it is necessary to code again after the moving picture data is decoded once and re-quantized again. Accordingly, it is difficult to quickly produce the moving picture data.
When the conventional moving picture coding apparatus is used and the rate is controlled by performing the re-quantization, it is difficult to conduct the rate control without causing the image quality deterioration in the next frame on which the re-quantization is conducted. This is due to motion compensation error.